Such transmissions essentially comprise a speed-reduction gear and a distribution gear, which has a ratio of I=1.
The speed-reduction gear decreases the high speed of the motor to one that the extruder can use. The speed-reduction gear in one two-shaft extruder transmission, known from European Exposure 716 914, is a spur gear.
Known two-shaft extruder transmissions are powered by a three-phase motor with a fixed speed. A gearshift integrated into the speed-reduction gear makes it possible to establish two fixed take-off speeds at the distribution gear. The speed required by the extruder at any particular moment can accordingly be at least discontinuously established. The range of adjustment R=required for the extruder's screws is rather narrow, approximately 1.1 to 1.2. In spite of this narrow range, establishing the speed at extruder outputs as high as 10 000 kW is difficult. A mechanical gearshift for a two-shaft extruder transmission at this range of output is accordingly not without problems. One alternative, continuous speed adjustment by way of the motor (a direct-current motor) is complicated and expensive at that range and would require an auxiliary drive mechanism for the requisite creeping speed.
Both of the take-off shafts in the two-shaft extruder transmission are accommodated in the distribution gear, allowing the extruder shafts to be turned in the same or in opposite directions. One take-off shaft is coupled directly to the speed-reduction gear. The high stress on the parts dictates that the take-off shafts be very close together, and the output must accordingly be branched, with the power needed by the second take-off shaft divided by spur gears between two distribution-gear shafts and then recombined. Due to this bifurcation, the cogs on the spur gear will only need to handle one quarter of the total power. Furthermore, two axial bearings in the distribution gear accommodate the very powerful axial forces deriving from the extrusion process.
The distribution gears in the two-shaft extruder transmission known from European Exposure 716 914) are spur gears. The distribution gear in a two-shaft extruder transmission known from German OS 3 734 578 is a planetary gear with the speed-reduction gear transmission stages next to it. Spur gears are employed for the transmission stages.
Another distribution gear, a spur gear, is known from German Patent 3 418 543. Its power take-up shaft is freely adjustable axially and has two oppositely cogged pinions.
Also known are "superimposed gears", which operate with two motors to obtain continuous speed adjustment without mechanical gearshifts. Two speed factors can be added or subtracted within a single planetary gear in accordance with the gear's inner ratio I.sub.o and with the direction of the sun wheel and ring.